This invention relates to a process and apparatus for removing heavy metal pollutants from aqueous waste water streams.
A common technique for removal of heavy metals from such streams is by the addition of hydroxide or sulfide to the streams to produce a precipitate. However, this precipitate forms a sludge which is difficult to dispose of because of the tendency of components of the sludge to become soluble in time which results in a serious leachate problem. Also, it is extremely difficult to process the sludge for recovery of soluble components in the same. Furthermore, the sulfide precipitation process evolves hydrogen sulfide, a noxious and toxic gas. Finally, neither of these processes are capable of providing the low residual concentrations called for in actual and pending legislation regulating the allowable concentration of heavy metals in domestic and industrial waste water.
Recently, an approach has been suggested for the removal of heavy metals from waste water streams by a starch xanthate process as set forth in the following papers: Wing, R. E., "Heavy Metal Removal from Waste Water with Starch Xanthate", paper presented at the 29th Purdue Industrial Waste Conf., Lafayette, Ind., May 7-9, 1974; and Wing, R. E., et al, "Insoluble Starch Xanthate: Use in Heavy Metal Removal", unpublished report, North Regional Research Laboratory, U.S.D.A., Peoria, Ill., August, 1974. In accordance with this process, the waste water is contacted with an insoluble starch xanthate formed by the reaction of a highly crosslinked starch in alkaline slurry with carbon disulfide. The metal ions in the waste water form an insoluble metal xanthate complex with the starch substrate which is stated in the papers to be separable from the waste water by filtration or other dewatering means. The papers also state that the heavy metals can be stripped from the starch by acid treatment resulting in metal recovery and that the starch can be rexanthated and recycled.
Experiments have been performed in accordance with the above starch xanthate process. However, it has been found that the starch xanthate residue from contact with the waste water stream forms a gelatinous mass which is extremely difficult to handle and dewater. Settling of the suspended material is slow and the supernatant liquids are turbid and cloudy. Upon filtration, the suspended material and residue form a cohesive mass on the filter paper which can only be removed by scrapping of the filter. This filter cake is of a sticky and gummy nature which is not conducive to ready handling on an industrial scale, especially if the material is to be rexanthated.
Another unexpected problem with the starch xanthate is that use in a wetted slurry form does not reduce the residual heavy metal content to the desired low level in contrast to use in dry form. However, drying of the starch xanthate after formation is an expensive procedure.
In view of the foregoing, it would be beneficial to provide an improved and efficient method of heavy metal removal from waste water based upon the xanthate complexing principle while avoiding the deficiencies of handling starch xanthate.